As I have said before here, the people who really know who is a good or bad tank, are the healers.
Having healed all types of tanks in all sorts of zones up to level 70, my experience is that it very much depends on both the player skill and equipment.
A well played Shadowknight with very good armour will usually be a much better tank than an "average" warrior.
However, if you are fortunate enough to find a skilled and attentive warrior with very good armour, then he/she will without question be the better tank.
The real test is when things go a bit wrong, I guess by now pretty much every one has done a few "Creators", so we have some common ground for comparison.
I have done many creators as main healer, both with my regular friends group and with many pick up groups.
As a cross section of the population, I would have to say that warriors now clearly have the better tanking capability. This is especially noticeable when, as I said, things go wrong, like having to tank two unslowed mobs at a time. (A Druid healer's nightmare
).
A decent warrior can handle this and keep the group alive and functioning.
From what I have seen a paladin has great difficulty surviving and maintaining aggro on both mobs. A shadowknight does a very good job of holding aggro, but needs way better than average gear to survive.
Having said that, in recent months I have healed several shadowknights and paladins who I would much prefer to group with than some of the warriors (who seem to have never bothered training taunt and seem to have never got around to handing in all their tomes
).
It is rather amazing to get to 70 and find that you are once again getting killed from healing aggro when you try to heal a warrior
a problem I thought I had seen the last of 20 levels ago.